Conventionally, a pneumatic tire including sipes, which are narrow slits, in blocks formed in a tread portion has been known.
In addition, in recent years, there has been a pneumatic tire including three-dimensional sipes each formed into a zigzag shape extending in the tire width direction, in the tire circumferential direction and in the tire radial direction (for example, refer to Japanese Patent Application Laid-open Publication 2004-203128).
In a pneumatic tire including such three-dimensional sipes as described above, the surface of each block is divided along the tire width direction. Accordingly, although the starting and braking performances on snow are improved, the rigidity of the block is made insufficient. This leads to a problem of an insufficient stability in straight running on the dry road surface.
For this reason, it has been tried to reduce the depth (the length in the tire radial direction) of the above-described three-dimensional sipes to improve the rigidity of the block so that the stability in straight running on the dry road surface can increase. However, in this case, as the block wears progressively, the depth of the three-dimensional sipes is further reduced, so that the rigidity of the block is excessively improved. As a result, a problem occurs that the starting and braking performances on snow are deteriorated.
In view of the above-described problems, an object of the present invention is to provide a pneumatic tire that achieves a highly good balance between the stability in straight running on the dry road surface and the starting and braking performances on snow.